This invention relates to well drilling rigs having improved jacking mechanisms for moving a well pipe vertically. The invention will be described primarily as utilized for lowering a string of casing into a well after completion of the drilling operation.
The casing string which is lowered into a well after drilling is often extremely heavy and difficult to support and control, and in many instances has a weight greater than the maximum load which the rig drawworks, traveling block, etc. are capable of suspending. In copending United States Patent Applications Ser. No. 169,718 entitled "Well Casing Jack Mechanism", filed July 17, 1980 by George I. Boyadjieff and Andrew B. Campbell, Inventors and United States Patent Application Ser. No. 268,763 entitled "Well Pipe Jack", filed June 11, 1981 by George I. Boyadjieff and Andrew B. Campbell, Inventors, there have been disclosed jacking mechanisms which can be positioned in the rig and utilized after completion of the drilling operation to lower a string of casing into the well. These jacking devices preferably include two piston and cylinder mechanisms located at different sides of the well axis and which actuate one of two pipe gripping units upwardly and downwardly relative to the second of those units to jack the casing downwardly in a step by step manner.